Note: Descriptions are shown in the official language in which they were submitted.
~ t~ No. L-5380 ~ 3 ~
:
... .
In the processing of molten aluminum, oxidation
forms aluminum dross, chiefly aluminum oxide with varied
amounts of impurities. The dross is skimmed from the molten
metal and is processed in rotary furnaces with fluxes of
sodium and potassium chlorides which are added to promote
dross separation and eliminate gas inclusion. The processed
dross is discharged as a waste ~hich contains chiefly aluminum
~; oxid~, water soluble salts such as sodium and potassium
chlorides9 substantial quantities of aluminum metal and
lesser quanti~i2s of impuri~ies such as aluminum carbide
and aluminum nitride~
The dross is further processed through aluminum
recovery plants which mill and screen the dross and recover
the coarse particles which are substantiall~ pure aluminum.
The processed aluminum dross (dross taîlings) is discharged
as an aqueous slurry from which the waste solids are settled
and then discharged to a storage area~
The impurities in the dross ~ailings have~ here-
.. ;.. . . . .
'20 to~ore, precluded utilization of the alumina in the tailings
and, consequently~ the dross tailings have accumulated over
the years in piles adjacent alaminum recovery plants. These - -
piles are unsightly and present environmental problems since
the water soluble salts in the tailings are leachea by rain
and are discharged to surface and ground waters~
Aluminum sulfate hasJ in recent years, been in
increasing demand~ principally for use as a floculant in
water clarification for industrial and sewage water treatment
plants. The aluminum sulfate is currently manu~acturea
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, t. No. L-5380 ~ 3 4
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by the reaction of sulfuric acid with valuable aluminum
sources such as aluminum oxide trih~dra~e and/or bauxite.
Although the aluminum dross tailin~s would appear to be
a less expensive source raw material for aluminum sulfate,
the contaminants present in the dross tailings ef~ectively
have prevented utilization of this waste material.
.. . .
.
This invention comprises a treatme~t of aluminum
dross tailings to decrease the roncentration of contaminants -
therein to acceptable leYels and permit utilization of alumina
.: in the dross tailings. This invention also comprises a
continuous flow process for the production of aluminum sulfate
from su~fur~c acid and aluminum oxides and, in particular,
~rom alumlnum oxide dross ta~ling~,
; 15 The aluminum dross tailings are treated in the
invention by washing, to remove water soluble salts from
an a~ueous slurry of raw dxoss tailings~ and reacting the
- dross contaminants with water at elevated temperatures~
The reaction is performed in the liquid phase and under
~20 sufficient agitat;on to insure particle-to-particle attrition
to break the protective aluminum hydroxide film-which forms
about the aluminum metal contaminant particles, thereby
achieving substantially cQmplete oxidation of the aluminum
metal contaminant to use~ul aluminum oxide trihydrate.
The conditions of the reaction are also sufficient to effect
the decomposition of any aluminum nitride contaminant which
ma~ be present to aluminum oxide trihydrate and ammonia
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Dkt. N~. l,-r~380
and/or the decomposition of any aluminum carblde contaJninant
which may be present to aluminum oxide trihydrate and methane
In the preferred processing, the reaction of the
contaminants with water is accelerated by additives which
destro~ the protective aluminum hydroxide ilm surrounding
the aluminum metal, such as trace quantities of caustic
which supplies h~droxyl lisands t~at complex and solubilize
the aluminum hydroxide and/or the use of inert, refractory,
particulate materials such as ceramic balls to mechanically
assist the attrition. In a preferred embodiment, the gases
evolved from the reaction are collected, dried ana burned
to supply energy requirements of th~ process~ In other
preferred embodiments7 the reaction is performed in a con-
: '
tinuous upflow clarification vessel to remove a slurry of
;~ 15 fine particles of aluminum oxide from the upper portion
r of the vessel from the more dense and larger fractions which. - are selectively concentrated with the contaminants and which
remain in a highly turbulent reaction ~one and are subjected
~'` . .
~ . ~herein ta a~trition.
Aspects of the invention are illustrated, merely by
example, in the drawings, in which:
ZO
, ~.," ,' . ' ' '"'' .'
. . FIGURE 1 is a schematic flow diagram oE.the pre
treatment process; ~~-
IGURE 2 is a graphical illustration of the rate
of reaction of typical contaminants at varied conditions;
. 25 FIGURE 3 is a schematic flow diagram oE a suitable
continuous flow aluminum sulfate process;
. . .
.; FIGURE ~ is an elevational view oE a suitable
'l~ mixing device for use in the aluminum sulate process; and
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Dkt. No. L~5380
FIGURE 5 illustrates a modification o~ the process
: diagram of FIGU~E 3.
:' .
Referring now to FIGURE 1, the aluminum dross
tailings are removed from the storage pile 10 by a clam
~ ~shell or shovel 12 and are passed to a suitable size reduction
:~ step such as a jaw crusher and/or ball mill 14. In balI
mill 14, the larger particles are reduced in size by tumbling
against other particles and against gravel and inert solids
1~ which are present. Water, preferably recycled brine, is
added to the ball mill 14 to form a slurry~ The oxides
; are removed from ball mill 14 as a slurry of from 5 to about
20 weight percent solids through line 16 and are passed
~hrough ~3a~eening ~perai~io~as ~uch a~. tro~unel scr~ 18 whiCh
. 15 . has a size of about 14 mesh where the largest size materials,
: comprising inerts such as rocks and gravel, are removed
for disposal through line 20. The ma~erials passing the
screen are passed as a slurry through line 22 to subsequen~
scrieenin~ in Sweco screen ~4 9 which has a size of about
~0 30 mesh, where the coarse oxides and some aluminum metal
are removed through line 26 and are returned to ball mill
14. Since the aluminum particles are generally larger thian .-
the aluminum oxide particles~ the oversize fraction removed
in line 26 will be enxiched in aluminum and all or a portion
~5 of the slurry in line 26 can be diverted to further screening
and classification to recover the valuable aluminim metal.
- The material~ which are of a suitably small sub-
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¦ Dkt. No. L-5380
d.ivision for processing through the subsequent stages, typi-
I cally those passing a 30 mesh screen, are passed as a slurry
`I through line 28 by slurry pump 30 to a bank ~ liquid-solid
separators generally indicated at 32 to wash the water soluble
salts from the oxide. The oxide slurry is processed through
.I the separators in countercurrent contact with fresh wash
; ¦ water which is introduced to the process through line 34.
:. Typically, the fresh wash water is introduced through line
34 to contact the solids discharged into the last of the
serially connected liquid-solid separators.
.,.
Each of the liquid-solid separators is a vessel
~:/ having a vertical, tubular main section 38 and a side outlet
. section 40 intersecting the main section at an angle of
~'.
: 45 to about 65 degrees and a conical bottom 42~ The incoming
~,
~; 15 slurry is introduced into the open, upper end of the main
. - section 38 of the first .separator 44 and the washed solids
. are removed through the bottom port of its conical section
420 ~he wash water to each succeeding separator is introduced
~: into the outlet of the conicai section 42 of the precedi~g
¦ 20 separat~r. The wash water is removed through the side outlets
.40 of each of the separators and is passed through lines
., ,
~ 36 and 37 to be introduced as ~he inlet wash water for the
; preceding stage of the serially connectea separatorsD In
: . a typical process, a plurality of such separators, e.g.,
: 25 four to about six separators are serially interconnected
to effect a substantial washing of the salts from the oxides.
:.
'; The wash water is- removed from the side outlet
40 of the second separator 45 and is passea through line
. 47 as a source of brine for. the size reduction stage, such
as ball mill 14. This brine has a concentration of abou~
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-j Dkt. No. L-5380
5 to 10 weight percent salts, typically sodium and potassium
chlorides.
A brine solution containing from 15 to about 30
weight percent salts is removed from side outlet 40 of the
first separator 44 and is passed through line 33 to the
salt recovexy unit 76. ~his brine also has a suspension
of finely divided solids, chiefly aluminum oxides which
are recovered in the succeeding treatment.
An al~ernative method for washing the oxide and
removing the salts comprises filtration of the slurry delivered
by pump 30 and washing oE the filtered solids. Suitable
e~uipment for this is a settling clariier or a travelling
bed filter which has a continuous filter belt that passes
over vacuum receivers to remove the salt brine frorn the
. ~
oxides which are transported by the belt and reslurried
in fresh water.
The slurry of the washed oxides is removed rom
. . ! .
~¦ the last separator 46 through line 48 and is passed by pump
50 to a reaction vessel 52. The slurry is heated to a suitable
; 20 reaction temperaturer eOg. ~ from 1~5 to about 225 F.
.~. by suitable means, such as steam, introduced through line
.
~ 5~ and/or other heating means such as indirec-t heat exchange~
,;~ The reactions of the contaminates which occur in vessel ~- -
;i 52 are exothermic and the concentrations of the contaminates ~`
in the slurry will usually be sufficient to supply the necessary
:.,
heat to maintain the reaction temperature. Accordinglyr
- -- after start-up, heating may be unnecessary and, in instances
with high concentrations of impurities, cooling may be necessary.
The slurry of solids is maintained at the reaction temperature
and i5 confined within the reaction vessel 52 under suitable
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Dkt. No L-5380
;
pressure, typically from atmospheric to about 25 psig, prefera~ly
from 5 to about 12 psig, to maintain the liquid phase.
The slurry within the reactor is agitated, preferably by
a mechanically driven propeller mixer 56 that is connected
to a drive shaft 58 extending to an externally moun-ted motor
60.
The impurities which are present in the aluminum
oxides react in vessel 52 as follows: -
12~2O O~v 2[Al(OH)3 3H20] -~ 3H2
A14C3 ~ 24H20 ~ 4[AltOH~3 3H20] ~ 3CH4
~' A12N2 ~ 12H2o ~ 2[Al(OH)3 3~2] ~ 2NH3
;l The chief impurity that is present in the aluminum
I oxides is metallic aluminum, which can comprise from 0.1
to about 25, usualiy from 1 to about 5 weight percent (dry
basis) of the solids present. The reaction o the metallic
~;~ aluminum with water, however, is inhibited by the formation
of a protective layer of aluminum h~droxide which envelopes
the aluminum metal particles. The invention achieves a
continuous and accelerated rate of reaction of the aluminum
; 20 metal with the water by agitation o the slurry with the
mechanically dxiven propeller 56, which achieves particle-
to-particle attrition that breaks the protective oxide film
and insures the continuance of the reaction. -
¦ The reaction can be further accelerated by additives
~¦ 25 to the reactor 52 Suitable additives include mechanical
`1 attrition aids such as refractory particulate matter, e.g.,
I ceramic balls, that are of sufficient density to resist
floatation and removal with the slurry from vessel 52.
The ceramic balls thus remain ;n the lower p~rtion of reactor
52 and in contact with the denser and larger solids which
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Dkt~ No. L-53~0
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also concentrate in the lower portion of the reactor; achieving
~ a high attrition on this size fraction of solids. A suitable,
.: commercially available source for t~is additive co}nprises
. spheres of aluminum oxide that are available in diameters
from 1/8 to about 2 inches. Such materials are particularly
` suited for use in the invention since attrition or abrasion
of these particles will not introduce any foreign material
into the aluminum oxide suspension.
An example of a chemical additive for vessel 52
~ 10 is an alkali metal hydroxide such as sodium, potassium,
`~ or lithium hydroxide. Preferably; sodium hydroxide is employed.
The hydroxide is employed at a low to trace concentrations,
~ typically from 0.01 to about 1.0 preferably from about 0.07 to about 0.15 percent, of the total weightO The alkali
metal hydroxide is added, as needed r as an aqueous solution
t from tank 62 and metering pump 64. The hydroxide does not
function as a reactant in the process and accordingly, low
~; . concentrations of the hydroxide can be used.. Instead, the
hydroxide functions primarily as a source of h~droxyl ligands
which complex the aluminum hydroxide and remove the protective
i............... aluminum hydroxide film about the metallic aluminum particles,
; thus accelerating the oxidation. . .
... . .
~ Preferably, the slurry is maintained within vessel
. , . . : ~ .
: 52 in an upflow condition by introducing the slurry into
25 the.vessel through a bottom inlet port and withdrawing a
. . .
: : slurry of processed oxide through a top port. The reactor
.~ . 52 thus serves as a clarifier to effect a density and/or
... size separation with the higher density and coarser fraction
.
recycling internally within vessel 52 and the lower density
,
30 and finer subdivided oxides being removed as the process
,~.,
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34
Dkt. No. L-5380
slurry through the top port. The latter are passed as a
slurr~ to transfer pump 66 which can deliver the slurry
as the reactant feed to the aluminum sulfate continuous
flow process of the invention.
, 5 The gas which is evolved by the reactions which
occur within vessel 52 is withdrawn through line 68. The
I gas can be passed to a suitable wet scrubber 70 where it
;¦ is contacted with a spray o water or dilute sulfuric acid
from line 148 to cool and condense ~ater and remove ammonia.
When the sas is cooled to about 100~ F. or less, it can
be used directly as fuel~ Otherwise, it is desirable to
; pass the gas to a drying tower 72 where i t contacts concen-
trated sulfuric acid from line 150 to reduce its moisture
content. The sulfuric acid in line 25 from drying tower
72 can be used as reactant in the aluminum sulfate process.
The driea gas can then be passed through line 74 as a source
of fuel for boiler 138 where it îs burne~ by air from line
140 to generate steam in tubes 142 from boiler Eeed water
introduced through line 144. The steam is used for the
- process, particularly for use in the salt recovery unit 76.
; The salt unit 76 comprises a plurality of treat-
ment stages to concentrate the brine removed from the solid
, . .
liquid separators through line 33 sufficientl~ to crystallize
the salts. Residual alumina fines can be removed by passing
the slurry through a settling vessel 78 to separate a clarifiea
-: :
brine from a fines concentrate.
The fines concentrate from vessel 78 is passed
to a suitable filter such as a felt filter~ or a rotar~
filt,~r 80. The alumina fines having a size range passing
a 200 mesh screen are separated as a filter cake on the
.. .
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¦ Dkt. No~ L-5380
''
drum ~4 of the filter and removed by a doctor blade ana
separa~ed through line 82. The alumina oxide is a relatively
pure product which can be passed by line 82 from filter
80 to the alum process or can be marlceted as an alumina
product. The Eiltered brine is removed interiorly of drum
84 and is passed to suitable concenkration treatment such
as solar evaporation ponds or vacuum evaporation. Preferably,
the concentration is effected by a multiple effect evaporator
; ~6. The steam generated in boiler 138 can be passed by
1~ line 146 to supply the steam required by the evaporatcr 86.
The concentrated salt liquor is removed from evapo-
~: rator 86 through line 88, passea through a cooler 90 and
to a vessel 92, where salt crystals are formed and removed
through line 94. The salt liquor is recycled to the evapo-
rator 86 through line 96. The salt crystals are passed
to centrifuge 98 to remove residual brine which is recycled
by line 100 and the salt crystals are dried by contact with
heated dry air. This contacting can be effected in equipment
such as a travellin~ bed conveyor 7 fluidi~ed bed, or auger
; 20 drier. Preferably, the crystals are dried in rotating drum
102 and removed through line 104.
Referring now to FIGURE 2, there is graphicall~ -
depicted the reaction rate of the aluminum metal particles
in a typical dross oxide raw material. The rate of reaction
i~ depicted as the amount of hydrogen evolved as a gas from
the reaction against the time of the measurement for the
reaction at varied temperatures of 185 F. and 210 F.,
respectively, curves 134, and 136, which were plotted Eor
reaction conditions otherwise described in the preceding
3~ example. In all instances, the slurry contained 0.5 weigh~
,!~'' . percent sodium hydroxide. Also depictea in FIGURE 2 is
: . ''
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34
¦ Dkt. No. L-5380
,~l curve 132 which illustrates the hydrogen evolution at 210~ F.,
. without the use of sodium hydroxide.
:. i
Referring now to FIGURE 3, there is illustrated
flo~ schematic of the preferred embodiment o~ the aluminum
S sulfate process. The reaction is conducted in reaction
vessel 11 which is supplied with thorougly blended and admixed
reactants from tubular mixer 21.
The sulfuric acid for the process is stored in
., . i
vessel 13 and is withdrawn through a positive displacement
metering pump 15 at a predetermined, constant flow rate.
~; The acid passed through line 17 to the central tube 19 of
tubular mixer 21. In the event of a flow blockage in line
. 17, a relief valve~23 is provided to bypass acid about pump 15.
: The aqueous slurry of pretreated aluminum dross
` 15 tailings is withdrawn from storage vessel 35 by pump 37
~; ~ at a flow rate controlled by valve 39. Alternativel~, pump
,~ 37 can be a positive displacement pump and the flow controlled
.` .by control of the speed of the pump. ~he aqueous suspension
~ ~ of alumina is dischar~ed through line 51 into the outer
.. ~ 20 tube of the tubular mixer 21 to proviae an annular, coaxial
,: flowing stream of slurry about the aci~ inlet tube 19.
The blended streams of acid and alumina-containing slurry
, . . .
are passed into the mixing portion 53 of tubular mixer 21 ..- -
:~ where the reactants are intimately admixed and wherein the
~ 25 exothermic heat of solution of the sulfuric acid is sufficient,
... : - with the preheat of the reactant streams, to raise the reactants-
,:
`. to an incipient reaction temperature~
. The reactants are discharged through riser 55
. : - into the upper portion of vessel 11 which is maintained
~; .,
partially filled with the reactants and which has a sufficient
- capacity to provide the desired residence time for substantially
,... . .
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Dkt~ No~ L-53~0
complete reaction. ~he reactants are maintained at super-
atmospheric pressure in vessel 11 by control valve 57 in
vent line 5~ which is controlled by pressure indicator controller
61 that is responsive to the measured pressure ~ithin the
vessel 11. This valve can also be opened periodically to
purge the sys.em of non-condensible gases such as hydrogen.
The purged gas can be passed to a vessel 119 packed with
inert solids, to trap any foam before venting to the atmosphere,
A relief valve 63 is provided and is set for relief of the
pressure at the predetermined maximum design pressure of
the reactor. The reaction pressure for a continuous process
is preferably the autogenic pressurP of the reactants.
It is possible, however~ to perEorm the reaction at greater
pressure by delivering the reac~ants to autoclaves at super-
-~ 15 atmospheric pressure or by injecting steam into the reaction
zone.
. . .
The reactants introduced into reactor 11 are main-
: tained at the optimum reaction temperature which ;s main-
;~ tained in the vessel by cooling of the reactants with tubular
heat exchanger 65~ Direct cooling by injectin~ a cold water
: .:
spray into vessel 11 can also be practiced, however r indirect
cooling is preferred, to avoid dilution of the reactants
in vessel 11. Cooling water is passed into khe heat exchanger-
".,.
65 at a rate controlled by valve 67 that is under control
, ~5 of temperatures at a number of locations such as thermocouples~
- 71 and 73v The heat exchanger 65 in vessel 11 is preferably
~ a bundle of Teflon tubes available from E.I. DuPont de Nemours
,' .
, ~ Company in preassembled units having a plurality of small
., ,
.' diameter Teflon tubes, typically, of a diameter of abou~
~ 30 0.05 to about 0.~ inch O.D~ This preassembled bundle is
,~ * Trade Marks.
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~` Dkt. No. L-S380
;
suspended in the reactor, and can be in the liquid or vapor
phase, or both, of the vessel. It is preferred to have
j the bundle in the vapor phase to insure adequate cooling
of the vapor phase and more precise control of the reactor
pressure than achieved when the bundle is in the liquid
I phase.
;¦ Th~ crude reaction product is withdrawn from vessel
11 through line'75 at a flow controlled by valve 93 and
' ' level controller 41. The product is diluted with water
and recycled aluminum sul~ate solution through line 77 at
';, , a rate sufficient to prevent solidification of the product
' and to maintain the optimum viscosity for succeeding product
.
' washing.
Th,e diluted product is passed to a plurality of
solid-liquid separators 79, 81, 83 and 85 which are similar
in construction and operation to the separators 32, previously
described. The aluminum sulfate solution is removed as
,~
;, , the product liquid stream through line 87 from the first'
separator. Solids from separator 79 are admixed with wash
:, 2C water from separator 83 and passed to separator 81. A dilute
aluminum'sulfate solution is removed through line 89 from
, separator 81 and passed to a surge tank lU5. A portion
of the solution is passed through control valve 103 and ''
,;,¦ line 77 to dilute the reactor product at a rate controlled ~~
by controller 101 that is responsive to the density of the
'',, product as detected by densitometer 69~ ' The balance of
~l - ' the dilute product can be recovered as a solution of 15-
,'~ 22% alum. Fresh water is added through line 91 in sufficientquantities to obtain the final produc~ concentration in
, 30 line 87. Fresh water can also be added throu9h line 65
',,,;~ to the product in line 75 to dilute the product sufficiently ' to avoid its solidification.
, ' ., ' ' , .
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Dkt. No. L-5380
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The rate of addition of the alumina ~o the process
. i5 controlled by the pump 37 and con~rol valve 39. Alter-
natively, a variable speed positive displacement pump could
be used to control the rate of alumina addition. This is
controlled by meter 33 which responds to conductivity sensor
41 which measures the sensed electrical conductivity of
," the reaction product. The conductivity sensor t generally
indicated at 43, is located in line 77 near the exit of
" vessel 11 to sense the conduc~ivity of the crude rea~tion
~, 10 product. It could be located at an intermediate elevation
within the reaction vessel 11. The conductivity meter that
is used in the preferred embodiment is available from the
~'' Beckman Instrument Company, Fullerton, Californiar and is
a magnetic i,nductance conductivity meter which has a sensing
.
lS element having no electrode exposed to the corrosive reactants.
-; The water for the process is supplied from tank
~" 107 to the fresh water header 109. Water from the heat
1 exchangers is returned through recirculation line 106 to
`~i* a suitable water cooler 113 that can be a heat exchanger,
~ 20 cooling tower or any conventional means for cooling of the
t return ~ater.
The water is returned to tank 107 through line
~" 115~ If desired, the heat of reaction from the process
;,; can be used to preheat the reactants b~ pass;ng a portion
~", 25 of the water from heat exchanger S5 to the slurry withdrawn
F from storage tank 35. This can be effected through bypass
", - line 117 and associated valving. The density of the slurry
c,,~ is controlled with a density sensor 31 such as a nuclear
,~ densitometer, controller 29 and control valve 27, which
, ' 30 regulates the addition oE hot water from line 117 or cold
'., ~ .
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Dkt. No. L-5380
;-.
water from line 109.
Referring now to FIGURE ~, there is shown a modi-
fication for the production of solid aluminum sulfate.
In this modification, the solution withdrawn from the reactor
11 is passed by line 111 to vessel 119 at a rate controlled
by valve 93 which func-tions as a shut-off valve. The product
; withdrawn from reac~or 11 is molten alum which can be flashed
to solid alum product. Preferably, this is accomplished
in vessel 119 where the alum is discharged as a spray by
a pxessure reduction valve 95. Alternatively, a droplet
forming nozzle and a spinner such as are conventionally
used in urea prilling towers could be mounted in vessel
119. The droplets of alum fall countercurrent to a dry
air stream supplied by blower 97 and solidify in vessel
119 from which they can be removed as alum prills by line
F
., gg-
~eferring now to FIGURE 4,-there is illustrated
a generally tubular mixer employed in the process. As illus-
; trated, the mixer has a reactank introduction section 121
bea~ing -end flanges 123 and 125 and a side nozzle 127 also
. ........ .
bearing an end flange 129. The introduction section 121
is of fiberglass construction and bears a coaxial internal
sleeve 19 having a base 133 for support on the interior
wall of section 121 and a plurality of axial ribs 135 at
its opposite end also for support of the tube 19. Flange
-~ 125 bears a closure plate 137 having a central bore through
which tube 19 extends. The acid supply line 17 is attached
. to tube 19 to provide introduction of the sulfuric acid
into the interior of the introduction section 121. The
side outlet 127 is attached to the line 51 for the supply
. ' ' ~
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of the aqueous slurry of the alumina-containing solid which
; is passed as an annular stream about the sleeve 19. The
blended streams are passed into the inlet of the tubular
:
` mixing section 53 that is secured to the end flange 123 of
section 121 by a retainer ring 141 and sealed thereto by
gasket or washer 143.
l; 3~
; The tubular mixing section 53 is of a design and
construction disclosed in U.S. Patent 3,286,992 issued
; November 22, 1966 to Parr, and contains a plurality of
lO curved sheet elements such as 145 and 147 (Figure 3) which
are axially positioned along the length of the mixing
:
~ section 53. The curved elements are formed of thin, flat
-~ sheet material having a width approximately equal to the
diameter of the tube and a length from 1.25 to 1.5 times its
width and twisted so that the upstream and downstream edges
` of each element are at a substantial angle to each other,
, .
~` e.g., at an included angle of from 60~ to about 210 F. As
; ,~
apparent from Figure 3, the adjacent elements are twisted in
opposite directions, e.g., element 145 having a left hand
.t'~
~ 20 spiral and element 147 having a right hand spiral in the
i direction of flow. In the translt through the mixing sec-
;,..il;
;~ tion, the blended reactants from the introduction section
~` 121 are caused to undergo a helical flow by curved elements
such as 145. The direction of rotation of this helical flow
is repeatedly reversed by the curved elements of opposite
rotation such as 147 so that the reactants are intimately
. ,.
admixed within a very short travel of mixing section 53.
'rhe sulfuric acid that can be used for the reaction
can be from about 30 to 99.4 weight percent sulfuric acid or
fuming sulfuric acid (oleum~ containing up to about 80
..;',
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Dkt. No. L-5380
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weight percent free sulfur trioxide. Generally, the acid
will be diluted to approximately 30 to 60 ~Jeight percent
in the reactor and, accordingly, acid o~ 30 weight percent
or greater can be employed in the reac~ion. The concentration
of the sulfuric acid supplied to the reaction depends on
the process technique. If the alumina-containing solid
is added as a solid to the reactor, all or any portion of
the necessary water can be added with the sulfuric acid,
eOg., sulfuric acid of a concentration as low as about 30
weight percent can be used. It is preferred, however, to
employ concentrated sul~uric acid and use the heat of dilution
of the acid as preheat for the reactants. Accordingly,
sulfuric acid of a strength above about 60 weight percent
is preferred. Concentrations of 70 to about 98 are preferred
and of 90 to about 99.4 weight percent are most preferred.
The reactants are heated sufficiently that upon
admixture with the reacting mixture of alumina and sulfuric
acid in the reaction zone they are heated to reacting temperature.
Preferably, the reactants are preheated to an incipient
reaction temperature of about 150 F., most preferably about
~ .
190 F., in the mixing zone and are in~roduced at that t~mperature
to the reactor. If desired, grea~er heating v the reactants
. , . . .
; can be achieved in the mixing zone and the reaction can
be initiated in the mixing zone to achieve a higher capacity
or throughput of the plant.
i~
The reactants can be maintained at a temperature
of from 225 F. to about 260 F. r however t a slightly higher
temperature range from 225 to about 350 F. and, most preferably,
about 285 to about 300 F. can be used in the reaction
zone to cbtain a high reaction rate. ~s previously indicated,
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this temperature is maintained by control of the flow r~te
of cooling water through the heat exchanger o~ the reaction
zone. The pressure on the reactants in the reaction zone
is the autogenic pressure of the reactants from 20 to about
;¦ 5 35 psig, preferably from 20 to about 120 psig and, most
preferably, from 39 to about 54 psi~, corresponding to the
aforementioned temperatures. The residence time of the
reactants in the reaction zone is maintained from 5 to about
45 minutes, preferably, from 5 to about 25 minutes and,
~- 10 most preferably, from 7 to about 20 minutes, and sufficient
to achieve from 78 to about 100, preferably, from 90 to
about 100 percent completion of the reaction. The product
removed from the reactor is diluted to provide a final product :.
having a aensity from about 1.2 to a~out 1.4, preferably,
~ 15 about 1.25 to about 1.35 specific gravity.. .~
The aluminum dross treatment will now be describea
- with reference to the following examples which will serve
to illustrate a mode of practice and demonstrate results
obtainable thereby.
' ~o Example 1
- Comparative laboratory experiments were performed
;l on samples of aluminum dross tailings having the following
:~: I . .. ..
sieve analysis:
~ Table 1 _
,:.1
Aluminum Metal
'¦ ~5Screen Mesh Size Weight Percent Concentration ~%?
+10 ~.7 100
~ l
~20 22.4 ---
: ~0 23.9 ___
; ~80 20.0 ---
-~200 15.9 ---
-200 13.3 --- `
-10 95.3 27.9
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; The aluminum dross tailings contained 20 weight
` ~ percent sodium and potassium chlorides. The dross tailings
were washed twice with fresh water, dried and processed
in a laboratory vessel having a magnetic bar stirrer. In
two successive experiments, 100 grams of dried dross-tailings
passing a 10 mesh screen were added to 186 grams water and
100 milliliters of ~lass beads of 1!8 inch diameter in the
;
laboratory vessel. The mixture was ~eated to and maintained
at 195-210 F. for two hours while stirring at 80 rpm.
n In the second experiment, 0.5 weight percent sodium hydroxide
~- was also added to the vessel contents.
After the reaction period of two hours, the vessel
: contents were removed, drained and dried and Table 4 summarizes
,.".,~
the results.
....
; 15 Table 2
Experiment Number 1 2
Aluminum Metal 1.7 - 0.2
Sieve Analysis
~2~ ~.1 0.2
` 20 ~4~ 3~g 3
,:.,,
f'',~'.' ' ', ~n 30æ 1.9
~2~0 - o.g 3~3
-200 ~ 87.4 ~1.6 - -;
~,~; Weight Dried Product145.9 149.0 ~~
; 25 Percent Conversion ~ -
f Al 90.7 99.2
< Example 2
., '
A laboratory investigation on the reactivity o
the aluminum metal in a typical aluminum oxide dross tailings
~ 3~ sample was studied in a stirred reactor. The reactor was
;¦ a metal vessel 14 inches high and 8 inches ;n diameter.
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Th~ vessel had an arcuate bottom and a propeller miY.er was
fabricated having a contour which conformed to the internal
shape of the vessel bottom. The propeller was suspended
.,.
in the vessel with a clearance of 3/8 inch between its edges
and the bottom wall of the reactor vessel. The vessel was
charyed in the experiments with three liters of water, one
liter of the dry aluminum oxide dross tailings (1300 grams3
¦ and one liter of aluminum oxide beads ~2093 grams~ having
a uniform diameter of 1/4 inch. The mixture was stirred
at a constant 70 rpm speed. The slurry within the vessel
'` was heated to a temperature o~ 196~212 F. by direct injection
of steam. The temperature was self sustaining by the reaction
exotherm during the first 60 minutes when water was added
: j to replace that lost by evaporation~ The reaction rate
i 15 decreased during the last 60 minutes and the temperature
was maintained during that period by resuming steam injection.
The dross tailings charged to the reactor were washed to
.~ .reduce their salt content to less than 0.5 weight percent.
The washed tailings were screened through a 14 mesh screen
` 20 and the -14 mesh fractionl which was used in experiment,
contained 28 weight percent metallic aluminum and had the
. .
; following particle size distribution:
. . .
,",................................................................... . ..
~ Table 3
:, . . .. ...
Screen ~esh Size Weight Percent ~-
+20 2.5 ~ - -
~40 13.5
;- ~80 62.1
+200 ~0.
-200 1.5
Af ter two hours, the reactor contents were discharged
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and dried and analyzed to determine that the metallic aluminum
content was 3.4 weight percent. The particle size analysis
of the product is set forth in the following table:
Table 4
5Screen Mesh Size Weight Percent
~20
.;
~40 2.8
~80 18.1
~200 28.5
-200
Exam~le 3
,l An aluminum dross treatment process such as illus-
trated in FIGURE 1 is operated for the production of 4628
weight parts of aluminum oxide trihydrate per hour. The
l 15 feed material which is introduced into the size reduction
: :~
step such as ball mill 14 comprises 4500 parts alumina trihydra~e,
r~, 5000 parts of a mixed potassium and sodium chloride salts
¢ and 500 weight parts aluminum. The preliminary screenings
:: .
~ result in separation of an enriched aluminum stream through
i 20 line 26 containing 300 weight parts aluminum per hour and
A 11 weight parts of potassium and sodium chloride in 33 weight
parts of water. The screen product which is passed by pump
30 to the solid-liquid separators comprises 200 weight parts ~- -
aluminum, 4500 weight parts alumina trihydrate, 5,951 weight - -
parts of mixed chloride salts and 17,853 weight parts water.
An equal amount of water in the quantity of 17,886 parts
per hour is added as wash water through line 34, producing
an oxide stream which is removed from the last liquid separatdr
through line 42 containing 200 weight parts aluminum, 4050
- 30 weight parts alumina trihydrate, 30 wei~ht parts mixed salts
.
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Dkt. NoO L-5380
~` and 2976 weight parts water. The recycle brine passed tnrough
line 47 to the milling operation comprises 962 ~7eight parts
mixed salts and 17,886 weight parts of water. The brine
;` removed through line 33 from the first separation stage
. .
contains 450 weight parts alumina trihydrate as fines passing
a 200 mesh screen, 4959 weight parts mixed salts and 14,877
parts of water. Fresh water in an amount of 3349 parts
`~ per hour is added to the washed alumina slurry passed through
. .
~ line 42 to the reactor 52. Caustic solution comprising
I ~:
44 parts water and 44 parts sodium hydroxide per hour is
introduced by pump 64 to blend wîth this stream passed to
~ reactor 52.
i' The reacted product removed from reactor 52 and
: .
passed by pump 66 to further reaction in the aluminum sulfate
plant comprises a slurry of 4628 parts alumina, 30 parts
mixed sodium and potassium chlorides and 44 parts sodium
hydroxide in 5,991 parts water.
~he concentrated brine is separated in thickness
~' ; 73 to obtain a clarified brine containing 4,678 parts of
mixed sodium and potassium chlorides in 14,034 parts per
hour of water which is passed to the multiple effect evaporator
86. The stream removed from the bottom of thickener 78
and passed to the rotary filter 80, comprises 450 parts
alumina hydrate, 381 parts of mixed sodium and potassium
chlorides in 844 parts per hour of water. This material
is filtered to recover the alumina fines as a slurry oE
450 weight parts alumina hydrate, 15 weight parts of mixed
salts in 300 parts water through line ~2. The filtrate
separated in filter 80 containing 4944 weight parts of sodium
and potassium chlorides in 15,578 weight parts per hour
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of water, is also passed to the multiple effect evaporator
where it is concentrated to obtain a concentrated solution.
The salts in the stream are recovered as dried product throug'n
line 104.
Example 4
A continuous process such as illustrated in FIGURE
r 3 is operated for the production of about 33,000 welght
parts of product per hour. The slurry is pretreated aluminum
oxide dross tailings is supplied to the reactant mixing
1~ zone at a rate of about 31 gallons per minute where ;t is
blended with sulfuric acid of about 98 weight percent con-
centration that is supplied thereto at the rate of about
9 gallons per minute. The admixed reactants during steady
state operation of the process have a temperature, resulting
from the exothermic heat of solution of the sulfuric acid
i upon mixing, of about 190 F. At the start up of the process,
steam i5 introduced into the slurry mixing tank to raise
."~, . .
,^ the temperature of the slurry to about 135 F. which is
suf~icient, with the heat of solution of the sulfur;c acia,
to heat the reactants upon mixing to a temperature o~ about
225 F, and thereby initiate the reaction.
The reactants are passed to ~he reaction zone
that is maintained at a temperature of 285-300 F. by circulation
of water throuyh its internal heat exchanger The reactor
pressure is maintained at 39-54 psig and the product is
withdrawn from the process at a rate sufficient to maintain
a 10 minute residence time within the reactor. The crude
reaction product withdrawn from the reactor is diluted with
water and dilute aluminum sulfate solution to produce a
final product having a density of 1.33, corresponding to
about 11 pounds aluminum sulfate solution per gallon.
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.~ The equipment used in the process is constructed
of suitable corrosion resistant material for handling of
the highly corrosive sulfuric acid reactants and the product
of the reaction. The tubular mixer of FIGURE ~ can be con-
. .
., *
structed of Pyrex or fiberglass reinEorced resins and the
reactor vessel can be constructed of fiberglass reinforcea
., ,~ . ,
resins with a Teflon heat exchanger bundle. The mixing
.: .
section of the tubular mixer can be constructed of glass
or, more preferably~ is also constructed of fiberglass rein-
-~ 10 forced resin for greater structural strength.
~ The invention has been described with reference
,. .
~ to the presently preferred and illustrated embodiment.
.,
It is not intended that the invention be unduly limited
by this description of pre~erre~ embodiments~ Instead,
i~ 15 it is intended that the invention be defined by the reagents,
method steps, and their obvious equivalents, set forth in
the following claims~
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. * TRADE MAl~K -
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